Different growth performance, lipid deposition, and nutrient utilization in in-season (S1) Atlantic salmon post-smolt fed isoenergetic diets differing in protein-to-lipid ratio

The aim of the present study was to evaluate how isoenergetic diets with different protein-to-lipid ratio affects feed intake, growth performance, lipid deposition, feed and nutrient utilization in Atlantic salmon post-smolt. A 6-month's feeding trial was conducted with in-season (S1) Atlantic salmon post-smolt reared in the sea under natural conditions (May–September). Quadruple groups of salmon (initial weight 95g) were fed two isoenergetic diet series formulated to contain a high (HP) and low (LP) protein-to-lipid ratio designed to resemble upper and lower levels of ratios used in commercial feeds. The group fed the HP diet had a significantly (P≤0.05) lower muscle fat content (HP=4.7%, LP=5.7%), whole body lipid (HP=9.0%, LP=9.6%) and energy content (HP=7.7MJkg−1, LP=8.0MJkg−1) than the group fed the LP diet after the period June–July. These differences were mainly due to significantly lower absolute apparent lipid retention in the summer period for post-smolt fed HP diet. In the subsequent experimental period (July–September), a significantly higher specific feed intake (HP=1.38%, LP=1.33%), thermal growth coefficient (HP=3.82, LP=3.46) and weight gain (HP=658g, LP=552g) were observed for fish fed the HP diet. The period from July–September was associated with high water temperatures and declining day length. The reduced feed intake in the LP group coincided with increased visceral mass and lipid deposition, indicating a possible involvement of lipostatic regulation. The retention efficiency of nutrients increased with the up-regulation in feed consumption. The HP fed fish had a significantly higher whole body lipid retention (HP=74.4%, LP=67.2%), but significantly reduced visceral mass compared to LP fed fish during the autumn. The overall improved growth, good protein utilization and reduced visceral adiposity among the HP fed fish resulted in significantly improved final condition factor (HP=1.46, LP=1.40), carcass yield (HP=86.0%, LP=84.1%), feed conversion based on gutted weight (HP=0.98, LP=0.93) and whole body protein (HP=17.6%, LP=16.9%). The present study reveals that low dietary protein-to-lipid ratios for salmon post-smolt may negatively affect production parameters, although digestible energy contents in the diets are similar.The present study confirms the importance of balanced dietary lipid-to-protein ratios for optimal production efficiency and nutrient utilization, and the significant effects of dietary and seasonal interaction on lipid deposition and production related parameters. To our knowledge, few have investigated the effect of isoenergetic diets differing in protein-to-lipid ratio on growth performance and nutrient utilization of juvenile Atlantic salmon reared in seawater under natural conditions. The experiment used feed formulations, fish breed and rearing conditions relevant for current commercial salmon farming practices.Considering the current increase in the cost of lipid sources, it would be beneficial for the aquaculture industry if dietary lipid content could be reduced without compromising growth and feed utilization of the fish. We believe our findings will provide useful and relevant information regarding dietary formulations and nutritional knowledge for the global fish feed industry and salmon producers.